All electronic communications systems suffer from the problem of background noise, regardless of whether the communication signals are analog or digital. It has always been the objective to improve the signal-to-noise (S/N) ratio in electronic communication systems, as background noise makes it more difficult to detect and/or distinguish the communication signal. If the background noise level is so strong that it obscures the transmitted signal, the receiver unit will not be able to detect the signal.
Known techniques for improving S/N ratio have included increasing the power of the transmitted signal, increasing the sensitivity of the receiver unit and using specially designed antennas, however, the capabilities of these techniques are limited. For example, there are situations where the transmitted signal power must be maintained at a relatively low level due to weight and/or power design constraints or the transmitter unit. Additionally, the design of the receiver unit, may be limited by size, weight and/or cost. These design constraints restrict application of higher sensitivity components in the receiver unit. Furthermore, the distance between the transmitter and receiver units may be so great so as to cause a significant degradation of the signal strength, regardless of the capabilities of either the transmitter or receiver units. Moreover, velocity changes between the transmitter and receiver units would also adversely affect the S/N ratio.
This is especially the case with the Navstar Global Positioning System (GPS) where the receiver unit is usually in motion while receiving signals from the satellite transmitters in orbit above the earth. Often, transmitted signals from GPS satellites undergo phase changes at the receiver unit due to unknown relative velocity changes as a result of unmeasured receiver unit accelerations and also as a result of phase-perturbing effects of the intervening ionosphere. In order to differentiate the transmitted signal from the noise, GPS satellites and receivers employ spread spectrum technology, wherein the phase of the transmitted signal is modulated digitally with a Pseudo-Random Noise Code. Thus, using correlation techniques, the receiver can detect and track the signal by matching the spread spectrum signal emanating from the satellite with an image of the signal generated in the receiver.
Such correlation techniques, however, are ineffective when the phase of the signal to be detected is changing in an unpredictable way during the period of time over which the detecting and tracking process occurs (integration time). The unpredictable phase may be due to any or a combination of several causes, including a variation caused by a changing propagation delay in the intervening medium between the transmitter and receiver. Unknown phase movement of the signal spoils the coherence between the locally generated template and the incoming signal and often limits processing gain. The output of the correlation procedure is typically proportional to cos (xcex94xcex8), wherein xcex94xcex8 is the phase error between the locally generated template and the incoming signal. For instance, a 60xc2x0 error reduces the correlation by 50%, and a 90xc2x0 error reduces the correlation to zero. Thus, the phase coherence must be maintained to approximately one-radian during the integration time interval, thereby leading to an upper limit on the coherent integration time which depends on signal phase predictability. If, for example, the uncertainty in relative velocity between the satellite transmitter and receiver unit is xcex94V, then the maximum integration time is c/(2xcfx80fxcex94V) where c is the speed of light and f is the carrier frequency. In the GPS for example, where the frequency of one of the carrier signals is 1.57xc3x97109 Hz and xcex94V is 1 m/sec, the integration time limit is 30 milliseconds (ms). Therefore, it can be seen that at higher frequencies, the integration time limit becomes more stringent, and therefore becomes a severe limitation in dynamic applications on moving vehicles where the velocity may be changing rapidly.
Thus, there is a need for enhancing the detection and tracking of transmitted signals or coded signals within a noisy background environment, whereby the effective coherent integration time is extended beyond what would otherwise be possible because of a lack of signal phase predictability. The present invention satisfies these needs, as well as others, and generally overcomes the deficiencies found in the background art.
The present invention is a method and apparatus for detecting and tracking coded transmitted signals in a noisy background environment. The method of the present invention, generally comprises the steps of correlating a signal in-phase and in-quadrature over subintervals of time and assigning a value to each in-phase and quadrature correlated signal for each time subinterval; storing the in-phase and the quadrature correlated signal values into a first memory; retrieving previously stored data representing path phase displacement indices from a second memory; forming in-phase correlation sums and quadrature correlation sums of the in-phase and quadrature correlated signal values, respectively; and estimating the true phase path of the signal by determining the correlation sums having the maximum absolute value.
The apparatus of the present invention generally comprises a correlator means, a first memory means, a second memory means, a correlation sums incrementor means and an estimating means.
The correlator means performs an in-phase and quadrature correlation of a transmitted signal over subintervals of time and generates a value for each time subinterval representing the in-phase correlation component and a value representing the quadrature correlation component. These in-phase correlation values and quadrature correlation values are stored in the first memory means, which is coupled to the correlator means. The second memory means stores indices that represent all possible phase paths for the transmitted signal. The correlation sums incrementor means is coupled to both the first memory means and the second memory means and forms sums of the in-phase correlated signal values and the quadrature correlated signal values over the total number of time subintervals received from the correlator means.
The estimating means is coupled to both the correlation sums incrementor and the second memory means and determines the true phase path of the signal using the indices from the second memory means and the correlation sums having the maximum absolute value.
A means for generating the indices that represent all possible phase paths for a signal is coupled to a second memory.
An object of the invention is to provide an apparatus for detecting and tracking coded signals in a noisy background environment.
Another object of the invention is to provide an apparatus for detecting and tracking coded signals in a high jamming environment.
Still another object of the invention is to provide an apparatus for detecting and tracking weak coded signals or low bandwidth communications signals.
Still another object of the invention is to provide an apparatus which extends the effective coherent integration time beyond what would be otherwise possible because of a lack of signal phase predictability.
Still another object of the invention is to provide a method for detecting and tracking coded signals in a noisy background environment.
Still another object of the invention is to provide a method for detecting and tracking coded signals in a high jamming environment.
Still another object of the invention is to provide a method for detecting and tracking weak coded signals or low bandwidth communication signals.
Still another object of the invention is to provide a method for extending the effective coherent integration time beyond what would be otherwise possible because of a lack of signal phase predictability.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.